Electronic, resonant sonic alarm devices are well-known and widely used to provide an inexpensive and effective means for signaling emergency conditions. These are available under the trademark Sonalert from P. R. Mallory of Indianapolis, Ind. and follow the teachings of my U.S. Pat. No. 3,277,465. See also my U.S. Pat. No. 3,447,151. (Both hereby are incorporated by reference.) These devices use an inexpensive transducer, which has a sharp natural resonant frequency. The drive circuit utilizes this natural frequency, of the transducer itself, to determine the frequency of the driving oscillating circuit.
In some high risk situations, it is most important that the transducer and the associated driving circuit be in proper condition to function. However, units manufactured at present, i.e., the Sonalert, do not contain circuitry for indicating such "ready" state. In those cases where the indication of a "ready" state is needed, a test button is required together with periodic manual testing.